This section is intended to introduce various aspects of the art, which may be associated with exemplary examples of the present techniques. This description is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Most raw natural gas extracted from the Earth contains methane (CH4), small amounts of other hydrocarbons and to varying degrees other compounds, i.e., contaminants. The methane component, as a low molecular weight hydrocarbon, is typically the desirable component within the raw natural gas. Compared to other carbon-based fuels, such as coal and oil, the burning of methane produces less carbon dioxide (CO2) emissions for each unit of heat released. That is, based on its ratio of heat of combustion to its molecular mass, methane produces more heat per mass unit than complex hydrocarbons. Further, methane is generally transported with ease. Thus, in many cities, methane is piped into homes for domestic heating and cooking purposes. In this context, methane is usually known as natural gas. In the form of a compressed natural gas, CH4 methane can be used as a vehicle fuel. Since natural gas is viewed as the preferred choice of fuel due to its advantages, the demand to provide effective techniques to separate and remove contaminants from raw natural gas has significantly increased.
The contaminants and impurities in the raw natural gas may include acid gas contaminants including CO2, hydrogen sulfide (H2S) and mercaptans, as well as nitrogen (N2), helium (He), water vapor, liquid water, and mercury. Such contaminants and impurities may lead to equipment malfunction, production failure, and product contamination, among other detrimental production issues. For example, the CO2 contaminant when combined with water may create a corrosive form of carbonic acid. Additionally, CO2 may reduce the BTU value of the natural gas and lower the economic viability of the natural gas when it is present, for example, in concentrations of more than 2 mole %.
During normal operations of a cryogenic distillation column, the CO2 contaminant, among other contaminants, may be separated and removed from the raw natural gas to produce a purified methane gas product. To start-up the cryogenic distillation column, before entering into normal operating conditions, various start-up techniques may be implemented. For instance, the start-up may be “assisted” by, for example, the use of solidification inhibitors or by using a clean methane flow to generate the initial liquid reflux during the start-up procedure. In contrast, unassisted start-ups generally do not involve the use of solidification inhibitors or clean methane. Instead, an “unassisted” start-up uses the raw natural gas that includes the contaminants, as described, and without solidification inhibitors, to generate the liquid reflux.
The aforementioned techniques may provide for start-up of a cryogenic distillation column where a clean liquid reflux may be generated for use during subsequent normal operating conditions. However, in the competitive business of cryogenic distillation and natural gas production, there remains an ongoing need for improved start-up techniques for cryogenic distillation systems.